In 2015 my team had Backbone, Angular, and React in a single app for a period of time. We merged two separate applications (Backbone and React) from two separate teams into a single application. We gradually moved the application over to entirely React over the course of about 7 months. not that uncommon.

At my previous job we started out with Angular in 2017. After some time passed benefits of using React was clear to everyone in the team. We wrote new features in React. The old Angular code worked as expected. It made little sense to spend time porting that to React.